This invention relates to a fluorescent display device adapted to impinge electrons emitted from an electron emission source such as a filamentary cathode, a field emission cathode (FEC) or the like on a phosphor to carry out desired display, and more particularly to a fluorescent display device in the form of a tube which facilitates mounting thereof on a circuit board (printed circuit board) on which various circuit parts are mounted and minimizes a variation in pitches of lead terminals and deformation thereof.
Now, a conventional fluorescent display device which has been widely known in the art will be described with reference to FIGS. 7 and 8.
The conventional fluorescent display device generally designated at reference numeral 1, as shown in FIG. 7, includes an envelope 2 which is formed into a box-like shape and has an interior hermetically kept at a high vacuum. The envelope 2 includes a lid-like casing 6 constituted by an insulating substrate 3, an insulating and light-permeable front cover 4, and an insulating frame-like side plate 5.
The substrate 3 of the envelope 2, as shown in FIG. 8, is formed on an inner surface thereof with a wiring layer 7 of a predetermined pattern corresponding to a display pattern 8 and wirings between segments or picture cells 10. The wiring layer 7 has an insulating layer 9 laminatedly formed thereon. The insulating layer 9 is formed by subjecting an insulating glass paste consisting of, for example, a lead borosilicate glass powder, a powder of an inorganic material such as a heat-resistant pigment or the like, and a vehicle to thick film printing. The insulating layer 9 is formed at a portion thereof corresponding to each of the segments or picture cells 10 of the display pattern 8 with a through-hole 11, through which the wiring layer 7 is exposed. The through-hole 11 through which the wiring layer 7 is exposed is closed with a conductor layer 12 by printing of a conductive paste.
The insulating layer 9, as shown in FIG. 8, is formed thereon with anode conductors 13, each of which is discretely allocated for every segment 10 of the display pattern 8 so as to be electrically connected through each of the conductor layers 12 to the wiring layer 7. The anode conductors 13 each are formed of, for example, a graphite paste consisting of a graphite powder and an inorganic binder or Al paste by thick film printing. The anode conductors 13 each have a phosphor layer 14 deposited thereon. The phosphor layers 14 each are made of a phosphor paste consisting of a phosphor powder and a vehicle by printing and formed into the same configuration as each of the picture cells or segments 10 of the display pattern 8. This results in the anode 15 being provided for every segment 10 of the display pattern 8. Arranged above each of the anodes 15 of the display pattern 8 is a grid or control electrode 16. The fluorescent display device 1 also includes filamentary cathodes 17 arranged above the grids 16.
The conventional fluorescent display device 1 constructed as described above is operated in such a manner that the filamentary cathodes 17 are driven for heating, resulting in thermions being emitted from the filamentary cathodes 17. Then, the thus-generated thermions are accelerated by the grids 16 having a positive voltage applied thereto while being controlled thereby, to thereby be impinged on the phosphor layer 14 of each of the anodes 15 positioned below the grids, resulting in the phosphor layer 14 being excited for luminescence, leading to desired display.
The fluorescent display device is mounted on a circuit board having various circuit parts mounted thereon. Such mounting, as shown in FIG. 9, is carried out by downwardly bending each of lead terminals 21 of various electrodes (anodes 15, grids 16, cathodes 17 and GNDs) led out of the envelope 2 in a horizontal direction along the substrate 3 by an angle of 90 degrees, inserting each of the lead terminals 21 in each of the through-holes 23 of the circuit board 22 electrically connected to the wiring pattern on a circuit board 22, and then fixing each lead terminal 21 in each through-hole 23 of the circuit board 22 by soldering in a reflow oven.
However, the above-described soldering in the reflow oven causes the fluorescent display device 1 to be introduced into the reflow oven being heated while being controlled, so that the fluorescent display device 1 is increased in temperature. This causes gas adhering to the cathodes 17 and phosphor layers 14 in the envelope 2 to be driven out thereof, resulting in a reduction vacuum in the envelope 2, leading to a deterioration in luminance of the fluorescent display device. In order to eliminate the problems, it is required to carry out an aging step of continuously driving the fluorescent display device 1 again. Mounting of the circuit parts on the circuit board 22 is carried out by surface mounting without introducing the circuit parts into the reflow oven, so that it is required to carry out soldering of the fluorescent display device 1 to the circuit board 22 in a step separate from that of mounting the circuit parts on the circuit board 22. This causes an increase in assembling cost of the fluorescent display device and a deterioration in operating efficiency.
Also, a fluorescent display device recently developed is generally increased in the number of lead terminals depending on desired display and reduced in pitches between the lead terminals in order to carry out complicated graphic display. This causes soldering in the reflow oven to be highly hard to form the lead terminal-inserting through-holes via the circuit board.
In view of the above, techniques of surface-mounting the fluorescent display device on the circuit board which are carried out without requiring formation of the through-hole via the circuit board are proposed as substitution for the above-described soldering in the reflow oven. The techniques proposed are practiced in such a manner as shown in FIG. 10. More particularly, lead terminals 21 of various electrodes including anodes 15, grids 16, cathodes 17 and GNDs each are led out of an envelope 2 of a fluorescent display device in a manner to horizontally extend along a substrate 3 and then bent at an intermediate portion thereof into an inverted L shape. Then, the lead terminals 21 each are formed at a distal end thereof with a substantially horizontal portion, which is provided thereon with a preliminary solder 24. Thereafter, the preliminary solders 24 each are arranged so as to face each of solder pads 25 positioned on connections on a wiring pattern of a circuit board 22. Then, soldering is carried out by means of a hot bar or manually between the solders 24 and the solder pads 25, resulting in the fluorescent display device 1 being mounted on the circuit board.
However, in the case that the fluorescent display device is so constructed that a lot of such lead terminals 21 are arranged and pitches between the lead terminals 21 are reduced, soldering by means of the hot bar causes short-circuiting between the lead terminals 21 adjacent to each other when the amount of solder used is increased. Also, a reduction in amount of solder leads to a failure in electrical connection between the connections on the wiring pattern and the lead terminals.
Also, manual soldering requires to solder the lead terminals 21 of the fluorescent display device to the circuit board one by one, resulting the soldering being not only time-consuming but highly troublesome.
In the prior art, when the fluorescent display device is to be shipped to a customer while being kept mounted on the circuit board, it is generally carried out to mount the circuit parts on the circuit board and then solder the lead terminals of the fluorescent display device to the circuit board in a last step. However, this causes mounting of the circuit boards on the circuit board and mounting of the fluorescent display device thereon to be carried out separately from each other, leading to a deterioration in operating efficiency. Thus, it is desired to concurrently carry out mounting of the circuit parts on the circuit board and mounting of the fluorescent display device thereon in a single step.
Also, the lead terminals are led out of the envelope. Such arrangement of the lead terminals leads to a variation in pitches between the lead terminals and damage to the lead terminals due to deformation thereof before and/or after mounting of the fluorescent display device on the circuit board, when pitches between the lead terminals are reduced and the lead terminals are formed into a thin shape.
Further, a variation or deviation in pitches between the lead terminals and deformation thereof likewise occur during a period of time for which the fluorescent display device is transported to a customer and then mounted on the circuit board in a site of the customer.
The present invention has been made in view of the foregoing disadvantage of the prior art.
Accordingly, it is an object of the present invention to provide a fluorescent display device which is capable of minimizing a variation in pitches between lead terminals of a fluorescent display device and deformation of the lead terminals.
It is another object of the present invention to provide a fluorescent display device which is capable of being positively mounted on a circuit board without soldering.
In accordance with the present invention, a fluorescent display device is provided. The fluorescent display device includes a box-like envelope having a vacuum held therein and including a substrate constituting a part thereof, various electrodes arranged in the envelope and having lead terminals led out of the envelope while horizontally extending on the substrate, a mounting holder arranged on a surface of the envelope opposite to a display surface thereof and having recess-like terminal holding holes formed at edges thereof at which side surfaces thereof and a bottom surface thereof intersect each other, and a fixing means for fixing the mounting holder and a circuit board on which the fluorescent display device is to be mounted to each other. The lead terminals each are bent at a distal end thereof so that the distal end is held in each of the terminal holding holes to hold the mounting holder therein, so that the fluorescent display device is mounted on the circuit board while keeping the mounting holder and circuit board fixed to each other through the fixing means and keeping the lead terminals pressedly contacted with connections on the circuit board.
In a preferred embodiment of the present invention, the terminal holding holes each are formed in a manner to correspond to a width of each of the lead terminals and a pitch thereof so that each of the lead terminals is received in each of the terminal holding holes.
Also, in a preferred embodiment of the present invention, the connections on the circuit board each are provided thereon with a solder pad, wherein the lead terminals each are soldered to each of the solder pads while pressedly contacting the distal end of each of the lead terminals with each of the solder pads.